1. Field of the Invention
The invention relates to intra-computer communications, to inter-device communications, and to buses.
2. Description of the Related Art
Buses provide an electrical, physical and logical interconnection for multiple peripheral devices of microprocessor based systems. Some bus systems are governed by protocols that align components in an hierarchical tree. Examples of such buses include the Peripheral Component Interconnect (PCI) bus, PCI-X (that is, PCI extended) bus, and the PCI-Express bus. The devices in the tree hierarchy typically communicate with the host processor and often times with each other. Devices may be separated on the tree by bridges and each bridge may communicate with the devices attached to it on a unique logical bus. Each bus within the tree may be assigned a unique number.
The tree structure and the bridges are bound by protocol rules that identify and process transactions on the bus. In the PCI protocol, transactions are classified as posted, non-posted and completions. Specific rules, commonly referred to as ordering rules, apply to the way the three transaction classes are passed through the bridge. Adherence to the ordering rules is required to guarantee system behavior.